In case you still doubted that dolphins have an awareness rivaling our own, new research shows that these cetaceans can remember the whistles of their old tank-mates after being separated for more than 20 years! No species other than humans has demonstrated such a long-term social memory.

In bottlenose dolphin communication, there are normal whistles, and then there are signature whistles. Every dolphin develops its own unique signature whistle, which functions sort of like a name. So when a dolphin comes into contact with another pod in the wild, for example, it will use its signature whistle as a way of announcing its identity. Recent research has also shown that bottlenose dolphins will call each other by their signature whistles, similar to the way we address each other by name.

Given that they live in a complex fission-fusion social system, where groups constantly disband and reform, bottlenose dolphins would certainly benefit from being able to remember each other's signature whistles for a long time. But dolphin memory hasn't exactly been the focus of a lot of research.

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"Dolphin long-term memory has been difficult to study," says Jason Bruck, a biologist with the University of Chicago. "Mostly we have anecdotal evidence of dolphins' long-term memory, such as how they can remember trained behavior for a few years."

What Bruck's interested in, however, is "social memory." Unlike semantic memory and episodic memory — which involve remembering facts and events, respectively — social memory deals with remembering other individuals. "In a social species, it may be the case that social memory is really important, but we don't know yet because it is poorly studied," Bruck tells io9. "We also don't know yet how it relates to episodic and other forms of memory."

To study the social memory of bottlenose dolphins, Bruck took a look at 43 individuals from six different facilities, including the Brookfield Zoo near Chicago, the Minnesota Zoo and The Seas at Walt Disney World. The dolphins were all part of a breeding program that rotated the animals between the institutions, allowing the dolphins to become familiar with different individuals over time.

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To start, Bruck took signature whistle recordings from all of the breeding consortium individuals and dredged up archived recordings of 20 additional dolphins. For his experiments, he habituated the dolphins to unfamiliar signature whistles — that is, he played the unfamiliar whistles through an underwater loudspeaker until the dolphins got bored and stopped paying attention.

He then played the signature whistle of a previous tank mate (the institutes kept records of which dolphins shared a tank and for how long). In response to hearing the whistles of their old friends, the dolphins would often quickly approach the loudspeakers, make eye contact with it, hover around it and even whistle at it. To try to test that the dolphin really was recognizing the whistle, Bruck then played a recording of an unfamiliar individual who was the same age and sex of the old tank mate.

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Bruck likens the experiment to what would happen if somebody projected a realistic hologram of someone you know in front of you as you walked down the street. "Dolphins are acoustic animals, so that's what they're perceiving," he says. "They may not necessary believe a dolphin really is there," but they can recognize the "image" of a familiar dolphin.

Overall, Bruck found that the dolphins responded significantly more to signature whistles from old friends than unfamiliar dolphins, no matter how long they were separated. In one case, two dolphins, named Allie and Bailey, had been separated for 20.5 years, but Allie still immediately recognized Bailey's whistle on the loudspeaker. In the wild, bottlenose dolphins live for an average of 20 years (though some hardy individuals can survive for nearly half a century), suggesting that the cetaceans have a lifelong social memory.

But this type of long-term memory may not be a dolphin- and human-only trait — it could be present in any species with a fission-fusion system. "Any time you see this complex social system in play, you have a good candidate for long-term social memory," Bruck says. For example, anecdotal evidence suggests that elephants, which live in a fission-fusion society, can also remember kin for decades. One of Bruck's next research projects will be to test the long-term social memory of chimpanzees. He's also interested in seeing if signature whistles evoke mental pictures of individuals in the dolphins' brains.

Whatever the case, the study suggests a link between sociality and cognition. Scientists often look at the evolution of advanced cognition from the standpoint of "Machiavellian intelligence," Bruck explains.

This notion means, essentially, that our deception, lying and trickery led to the evolution of greater intelligence. But those features don't seem to be "present in elephants and other intelligent animals," he says.

Instead, we may owe our advanced cognition to our fission-fusion way of life. "It is possible that our social system as humans might have facilitated the need to remember things in the long-term," Bruck says. "That might explain some of our own cognitive evolution."